Fix TerminalLogger hiding Exec output

[minaxi98]

Set StandardOutputImportance="high" so dotnet-getdocument output is visible. Fixes #62273

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Summary of the changes (Less than 80 chars)

Description

Root cause: The Exec task in Microsoft.Extensions.ApiDescription.Server.targets:67 logged stdout at default Normal importance. The .NET 8 TerminalLogger suppresses Normal-importance messages.

Fix: Added StandardOutputImportance="high" to the Exec task so the dotnet-getdocument.dll output (e.g., Generating document named 'v1') is elevated to high importance, which the TerminalLogger surfaces by default.

README update: Added a section documenting Terminal Logger behavior and the -tlp:v=d flag for more verbose output.

Fixes #62273

[dotnet-policy-service]

Thanks for your PR, @minaxi98. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[minaxi98]

@dotnet-policy-service agree

[dotnet-policy-service]

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